Well treatment method

ABSTRACT

Gelatin capsules containing oil field chemicals, preferably weighted with a heavy metal compound, are made more stable under certain conditions by the incorporation of a strong chelating agent. The microcapsules can provide an extended treatment period since many materials that would damage the microcapsules are controlled by the chelating agent.

[0001] This invention relates to a method of treating a hydrocarbon wellwith well treatment chemicals, in particular by down-hole placement ofpolymeric particles carrying well treatment chemicals or precursors orgenerators thereof, and to such particles and compositions andstructures containing them.

[0002] During the operation of a hydrocarbon well (i.e. a gas or oilwell) various down-hole problems arise such as corrosion of metalfittings, hydrocarbon flow-inhibiting deposition (e.g. of scale, gasclathrates, metal sulphides, waxes, gel polymers, microbial debris,etc.), generation of toxic hydrogen sulphide by sulphate-reducingbacteria, increased water flow into the producer bore, etc.

[0003] Thus, for example, where sea water is injected through aninjection bore hole into an oil-bearing stratum to drive oil through theformation (i.e. the rock) into the producer well hole, differences insolutes in the injection water and the water already present in theformation can cause metal salts to precipitate as scale so causinggradually increasing clogging of the producer well hole.

[0004] Typically this is dealt with by applying a “squeeze” of scaleinhibitor chemicals, i.e. chemicals which break down the scale andincrease oil or gas flow. This generally involves ceasing hydrocarbonflow, forcing an aqueous solution of the scale inhibitor down theproducer bore under pressure to drive the inhibitor solution into theformation, and restarting production. Such treatment generally allows afurther six or so months of hydrocarbon flow before a further squeeze isrequired and each squeeze causes some damage to the formationsurrounding the producer bore hole and as a result an increased flow offormation fragments (i.e. rock grains etc.) into the bore.

[0005] The producer bore hole in an oil well is generally lined in thehydrocarbon bearing stratum with “gravel packs”, sand containing filterelements, which serve to trap formation fragments and it has beenproposed to include in such gravel packs ceramic particles coated withor impregnated with well treatment chemicals such as scale inhibitors(see EP-A-656459 and WO 96/27070) or bacteria (see WO 99/36667).Likewise treatment of the formation surrounding the producer well borehole with well treatment chemicals before hydrocarbon production beginshas also been proposed, e.g. in GB-A-2290096 and WO 99/54592.

[0006] Various polymeric, oligomeric, inorganic and other particulatecarriers for well treatment chemicals are also known, e.g. ion exchangeresin particles (see U.S. Pat. No. 4,787,455), acrylamide polymerparticles (see EP-A-193369), gelatin capsules (see U.S. Pat. No.3,676,363), oligomeric matrices and capsules (see U.S. Pat. No.4,986,353 and U.S. Pat. No. 4,986,354), ceramic particles (see WO99/54592, WO 96/27070 and EPA-656459), and particles of the welltreatment chemical itself (see WO 97/45625).

[0007] Particles coated with well treatment chemicals, particlesencapsulating well treatment chemicals in a soluble shell and porousparticles impregnated with well treatment chemicals may however have theinherent problem that release of the well treatment chemical will takeplace relatively rapidly once the particles encounter water down-hole.Accordingly the protection they provide may be relatively short lived.

[0008] There is thus a continuing need for a means of well treatmentthat will provide prolonged protection.

[0009] It is therefore proposed to intimately incorporate a welltreatment chemical or precursor or generator within a polymeric matrixby effecting polymerization in a discontinuous aqueous phase containingthe chemical, precursor or generator, i.e. by an inverse suspensionpolymerization. In this way the rate of leaching of the chemical,precusor or generator may be controlled or selected by choice ofparticle properties (e.g. swellability, porosity, degradability, size,molecular weight, degree of crosslinking, etc.) which in conjunctionwith properties of the down-hole environment (e.g. temperature, pH,salinity etc.) will govern the leaching or release rate.

[0010] Thus viewed from one aspect the invention provides a method forthe treatment of a hydrocarbon well which method comprises administeringdown said well inverse suspension polymerized polymer particles havingimmobilized therein a well treatment chemical or a precursor orgenerator thereof.

[0011] By inverse suspension polymerized is meant that thepolymerization occurs in or on the surface of a discontinuous aqueousphase present in a continuous non-aqueous phase. Thus the polymerizationmay for example be an emulsion, dispersion or suspension polymerization.

[0012] The aqueous phase in the polymerization will contain the welltreatment chemical, precursor or generator in dissolved or dispersedform and will preferably also contain a water-soluble polymerizationinitiator. The continuous non-aqueous phase will desirably contain apolymerization inhibitor.

[0013] The polymer will desirably be a homo or copolymer of one or morewater soluble or amphiphilic monomers, e.g. monomers carryinghydrophilic moieties such as acid, hydroxy, thiol or amine groups.Suitable monomers thus include 2-acrylamide-2-methylpropanesulphonicacid (AMPS), methyl methacrylate (MMA), maleic acid anhydride (MAH), andother water-soluble monomers.

[0014] Polymerisation may be effected using conventional techniques,conditions, initiators, etc.

[0015] Viewed from a further aspect the invention provides inversesuspension polymerized polymer particles having immobilized therein awell treatment chemical or a precursor or generator thereof.

[0016] Viewed from another aspect the invention provides the use for themanufacture of hydrocarbon well treatment compositions of inversesuspension polymerized polymer particles having immobilized therein awell treatment chemical or a precursor or generator thereof.

[0017] Viewed from a still further aspect the invention comprises ahydrocarbon well treatment composition comprising a carrier liquidcontaining inverse suspension polymerized polymer particles havingimmobilized therein a well treatment chemical or a precursor orgenerator thereof.

[0018] Viewed from a yet further aspect the invention comprises atubular filter for down-hole placement containing inverse suspensionpolymerized polymer particles having immobilized therein a welltreatment chemical or a precursor or generator thereof.

[0019] In the method of the invention the polymer particles may beplaced down hole before and/or after hydrocarbon production (i.e.extraction of oil or gas from the well) has begun. Preferably theparticles are placed down hole before production has begun, especiallyin the completion phase of well construction.

[0020] The particles may be placed within the bore hole (e.g. in thehydrocarbon bearing strata or in ratholes) or within the surroundingformation (e.g. in fissures or within the rock itself). In the formercase, the particles are conveniently contained within a tubular filter,e.g. a gravel pack or a filter structure as disclosed in EP-A-656459 orWO 96/27070; in the latter case, the particles are preferably positionedby squeezing a liquid composition containing the particles down the borehole. Preferably, before production begins the particles are placed bothwithin the bore in a filter and within the surrounding formation.

[0021] Where the particles are placed within the surrounding formation,the pressure used should be sufficient to cause the particles topenetrate at least lm, more preferably at least 1.5 m, still morepreferably at least 2 m, into the formation. If desired, the particlesmay be applied in conjunction with proppant particles (e.g. as describedin WO 99/54592) to achieve a penetration of up to about 100 m into theformation. Compositions comprising proppant particles and polymerparticles according to the invention form a further aspect of theinvention.

[0022] The particles according to the invention advantageously have modeparticle sizes (e.g. as measured with a Coulter particle size analyser)of 1 μm to 5 mm, more preferably 10 μm to 1000 μm, especially 250 to 800μm. For placement within the formation, the mode particle size ispreferably 1 to 50 μm, especially 2 to 20 μm. For any particularformation, formation permeability (which correlates to the pore throatsizes in the formation) may redily be determined using rock samplestaken during drilling and the optimum particle size may thus bedetermined. If the particles have a very low dispersity (i.e. sizevariation), a highly uniform deposition and deep penetration into theformation can be achieved. For this reason, the particles preferablyhave a coefficient of variation (CV) of less than 10%, more preferablyless than 5%, still more preferably less than 2%.

[0023] CV is determined in percentage as${CV} = {100 \times \frac{{standard}\quad {deviation}}{mean}}$

[0024] where mean is the mean particle diameter and standard deviationis the standard deviation in particle size. CV is preferably calculatedon the main mode, i.e. by fitting a monomodal distribution curve to thedetected particle size distribution. Thus some particles below or abovemode size may be discounted in the calculation which may for example bebased on about 90% of total particle number (of detectable particlesthat is). Such a determination of CV is performable on a Coulter LS 130particle size analyzer.

[0025] For placement in filters, the particles preferably have modeparticle sizes of 1 to 5000 μm, more especially 10 to 1000 μm, stillmore preferably 250 to 800 μm. In such filters, the particles preferablyconstitute 1 to 99% wt, more preferably 2 to 30% wt, still morepreferably 5 to 20% wt of the particulate filter matrix, the remainingmatrix comprising particulate oil- and water-insoluble inorganicmaterial, preferably an inorganic oxide such as silica, alumina oralumina-silica. Particularly preferably, the inorganic oxide has a modeparticle size which is similar to that of the polymer particles, e.g.within 20%, more preferably within 10%. As with the in-formationplacement, the polymer particles preferably have low dispersity, e.g. aCV of less than 10%, more preferably less than 5%, still more preferablyless than 2%. The low dispersity serves to hinder clogging of thefilters.

[0026] The particles are preferably particles having a polymer matrixcontent of 5 to 95% wt, more preferably 30 to 90% wt, especially 40 to80% wt.

[0027] Preferably the polymer matrix of the particles has a softeningpoint above the temperatures encountered down hole, e.g. one above 70°C., more preferably above 100° C., still more preferably above 150° C.

[0028] The well treatment chemicals or precursors or generators thereofwhich the particles contain may be any agents capable of tackling downhole problems, such as corrosion, hydrocarbon flow reduction, or H₂Sgeneration. Examples of such agents include scale inhibitors, foamers,corrosion inhibitors, biocides, surfactants, oxygen scavengers, bacteriaetc.

[0029] The material which the particles contain may be a well treatmentchemical itself or a precursor chemical compound which in situ willreact, e.g. break down, to produce a well treatment chemical, oralternatively it may be a biological agent, e.g. an enzyme or bacteriumwhich produces a well treatment chemical which exerts its effect withinor outside the bacterial cells.

[0030] In general, the well treatment chemicals will be oil-insolubleand water-soluble chemicals which leach out of the polymer particleswhen water begins to reach the bore hole or the area of the formation inwhich the particles are placed. Where the particles contain welltreatment chemical generating bacteria, these are preferablythermophilic bacteria which in the absence of water are in a dormantphase, and especially preferably they are ultra microbacteria ornanobacteria. Generally where the particles contain bacteria, they willalso be impregnated with nutrients for the bacteria, e.g. sucrose, sothat bacterial growth is promoted once the particles encounter water.

[0031] Example of typical well treatment chemicals, precursors andgenerators are mentioned in the patent publications mentioned herein,the contents of all of which are hereby incorporated by reference.

[0032] Thus for example typical scale inhibitors include inorganic andorganic phosphonates (e.g. sodium aminotrismethylenephosphonate),polyaminocarboxylic acids, polyacrylamines, polycarboxylic acids,polysulphonic acids, phosphate esters, inorganic phosphates, polyacrylicacids, inulins (e.g. sodium carboxymethyl inulin), phytic acid andderivatives (especially carboxylic derivatives) thereof, polyaspartates,etc.

[0033] Where the scale inhibitor is a polymer it may of course containresidues of one or more different comonomers.

[0034] Examples of preferred well treatment chemicals include: hydrateinhibitors, scale inhibitors, asphaltene inhibitors, wax inhibitors andcorrosion inhibitors. Such inhibitors are well known to those working inthe fields of well treatment.

[0035] Where the polymer partices are placed within the formation, theyare preferably applied as a dispersion in a liquid carrier. For pre- andpost-completion application, the liquid carrier preferably comprises anon-aqueous organic liquid, e.g. a hydrocarbon or hydrocarbon mixture,typically a C₃ to C₁₅ hydrocarbon, or oil, e.g. crude oil. For curativetreatment, i.e. after production has continued for some time, the liquidcarrier may be aqueous or non-aqueous.

[0036] The invention will now be described further with reference to thefollowing non-limiting Examples:

EXAMPLE 1

[0037] Nitrilotrismethylenetriphosphonic Acid Containing Particles

[0038] 6.0 g of a 51.3% wt aqueous solution of AMPS, 0.16 g of1,2-dihydroxyethylenebisacrylamide, 3.0 g ofnitrilotrismethylenetriphosphonic acid, 0.3 g of acrylamide, 2.4 g ofdistilled water and 125 μL of a 0.2M aqueous solution of ammoniumpersulphate are mixed at 20° C. to produce an aqueous mixture.

[0039] 50 mL of toluene and 1.0 g of calcium stearate are mixed togetherand then nitrogen is bubbled through at 20° C. to produce a non-aqueousmixture.

[0040] The aqueous mixture is dispersed in the non-aqueous mixture at20° C., the dispersion is heated to 60° C. and kept at that temperaturefor 40 minutes.

[0041] 125 μL of a 0.2M aqueous solution of sodium metabisulphite isadded and the reaction mixture is kept at 60° C. for a further 2 hours.

[0042] The polymer particles are then recovered.

EXAMPLE 2

[0043] Nitrilotrismethylenetriphosphonic Acid Containing Particles

[0044] 6.0 g of a 51.3% wt aqueous solution of AMPS, 0.163 g of1,2-dihydroxyethylenebisacrylamide, 2.0 g of acrylamide and 2.4 g ofdistilled water are mixed together at 20° C. is 1.5 g ofnitrilotrismethylenetri-phosphonic acid is dissolved in 1.5 g distilledwater and this and 125 μL of a 0.2M aqueous solution of ammoniumpersulphate are added to the AMPS solution to produce an aqueousmixture.

[0045] 50 mL toluene, 4.0 g acetone, and 0.1 g calcium stearate aremixed and nitrogen is bubbled through at 20° C. to produce a non-aqueousmixture.

[0046] The aqueous mixture is dispersed in the non-aqueous mixture at20° C.

[0047] 125 μL of a 0.2M aqueous solution of sodium metabisulphite isadded, the temperature is raised to 60° C. for two hours.

[0048] The polymer particles are then recovered.

EXAMPLE 3

[0049] Particles Containing Triphosphonate

[0050] 3.6 g of sorbitan sesquioleate dissolved in a mixture of 108 mLtoluene and 42 mL chloroform are placed in a flanged 250 mL flask havinga temperature control blanket, a thermostat-controlled stirredcirculatory water bath and a spiral stirrer. The solution is stirred at160 rpm for 30 minutes at 4° C. under an argon atmosphere. 1.125 mL oftetramethylenediamine is added and stirring is continued for 10 minutes.

[0051] Argon is passed through a mixture of 4.2 g acrylamide, 0.21 gN,N′-methylene bis acrylamide, 0.23 g EDTA titriplex II, 22.5 mL of anaqueous pH 7.4 phosphate buffer (0.0087M KH₂PO₄ and 0.0184M K₂HPO₄) and4.0 g of a pH 7.4 triphosphonate solution for 2 minutes and then thesolution is added to 1.5 mL of an ammonium persulphate solution(produced by mixing 0.5 g ammonium persulphate and 10 mL of water). Theresulting mixture is rapidly added to the solution in the flanged flaskwhich is stirred for a further 80 minutes at 4° C.

[0052] The resulting polymer particles are separated from the reactionmixture, added to 75 mL toluene and then stirred using a magneticstirrer. The toluene is removed and replaced by fresh toluene andstirring is repeated. The toluene is again removed and replaced by atoluene/methanol mixture and stirring is repeated. The toluene/methanolmixture is removed and replaced by methanol and stirring is repeated.The methanol is removed and the particles are dried in air at ambienttemperature and then under vacuum at 40° C. for about 1 hour.

1. A method for the treatment of a hydrocarbon well which methodcomprises administering down said well inverse suspension polymerizedpolymer particles having immobilized therein a well treatment chemicalor a precursor or generator thereof.
 2. A method as claimed in claim 1wherein said polymer particles are administered before hydrocarbonproduction from said well is begun.
 3. A method as claimed in either ofclaims 1 and 2 wherein said polymer particles are placed in a filter inthe bore-hole of said well and in the formation surrounding saidbore-hole.
 4. A method as claimed in any one of claims 1 to 3 whereinsaid polymer particles are particles as claimed in any one of claims 6to
 9. 5. Inverse suspension polymerized polymer particles havingimmobilized therein a well treatment chemical or a precursor orgenerator thereof.
 6. Particles as claimed in claim 5 produced bypolymerization of a continuous non-aqueous phase and of a non-continuousaqueous phase, said aqueous phase containing said well treatmentchemical or precursor or generator thereof.
 7. Particles as claimed ineither of claims 5 and 6 wherein said polymer is a homo or copolymer ofAMPs, MMA or MAH.
 8. Particles as claimed in any one of claims 5 to 7whereof the polymer content is 30 to 90% wt.
 9. Particles as claimed inany one of claims 5 to 8 containing a well treatment chemical orprecursor or generator thereof selected from scale inhibitors, corrosioninhibitors, wax inhibitors, hydrate inhibitors, asphaltene inhibitors,foamers, biocides, surfactants, oxygen scavengers and bacteria.
 10. Ahydrocarbon well treatment composition comprising a carrier liquidcontaining inverse suspension polymerized polymer particles havingimmobilized therein a well treatment chemical or a precursor orgenerator thereof.
 11. A composition as claimed in claim 10 comprisingpolymer particles as claimed in any one of claims 6 to
 9. 12. A tubularfilter for down-hole placement containing inverse suspension polymerizedpolymer particles having immobilized therein a well treatment chemicalor a precursor or generator thereof.
 13. A filter as claimed in claim 12containing polymer particles as claimed in any one of claims 6 to
 9. 14.The use for the manufacture of hydrocarbon well treatment compositionsof inverse suspension polymerized polymer particles having immobilizedtherein a well treatment chemical or a precursor or generator thereof.15. Use as claimed in claim 14 of polymer particles as claimed in anyone of claims 6 to 9.